The invention relates generally to an electrophysiological ("EP") apparatus and method for providing energy to biological tissue, and more particularly, to a steerable catheter with a preformed distal shape for positioning the catheter to a desired location in a patient.
The heart beat in a healthy human is controlled by the sinoatrial node ("S-A node") located in the wall of the right atrium. The S-A node generates electrical signal potentials that are transmitted through pathways of conductive heart tissue in the atrium to the atrioventricular node ("A-V node") which in turn transmits the electrical signals throughout the ventricle by means of the His and Purkinje conductive tissues. Improper growth of, or damage to, the conductive tissue in the heart can interfere with the passage of regular electrical signals from the S-A and A-V nodes. Electrical signal irregularities resulting from such interference can disturb the normal rhythm of the heart and cause an abnormal rhythmic condition referred to as "cardiac arrhythmia."
While there are different treatments for cardiac arrhythmia, including the application of anti-arrhythmia drugs, in many cases ablation of the damaged tissue can restore the correct operation of the heart. Such ablation can be performed by percutaneous ablation, a procedure in which a catheter is percutaneously introduced into the patient and directed through an artery to the atrium or ventricle of the heart to perform single or multiple diagnostic, therapeutic, and/or surgical procedures. In such case, an ablation procedure is used to destroy the tissue causing the arrhythmia in an attempt to remove the electrical signal irregularities or create a conductive tissue block to restore normal heart beat or at least an improved heart beat. Successful ablation of the conductive tissue at the arrhythmia initiation site usually terminates the arrhythmia or at least moderates the heart rhythm to acceptable levels. A widely accepted treatment for arrhythmia involves the application of RF energy to the conductive tissue.
In the case of atrial fibrillation ("AF"), a procedure published by Cox et al. and known as the "Maze procedure" involves continuous atrial incisions to prevent atrial reentry and to allow sinus impulses to activate the entire myocardium. While this procedure has been found to be successful, it involves an intensely invasive approach. It is more desirable to accomplish the same result as the Maze procedure by use of a less invasive approach, such as through the use of an appropriate EP catheter system.
There are two general methods of applying RF energy to cardiac tissue, unipolar and bipolar. In the unipolar method a large surface area electrode; e.g., a backplate, is placed on the chest, back or other external location of the patient to serve as a return. The backplate completes an electrical circuit with one or more electrodes that are introduced into the heart, usually via a catheter, and placed in intimate contact with the aberrant conductive tissue. In the bipolar method, electrodes introduced into the heart have different potentials and complete an electrical circuit between themselves. In the bipolar method, the flux traveling between the two electrodes of the catheter enters the tissue to cause ablation.
During ablation, the electrodes are placed in intimate contact with the target endocardial tissue. RF energy is applied to the electrodes to raise the temperature of the target tissue to a non-viable state. In general, the temperature boundary between viable and non-viable tissue is approximately 48.degree. Centigrade. Tissue heated to a temperature above 48.degree. C. becomes non-viable and defines the ablation volume. The objective is to elevate the tissue temperature, which is generally at 37.degree. C., fairly uniformly to an ablation temperature above 48.degree. C., while keeping both the temperature at the tissue surface and the temperature of the electrode below 100.degree. C.
Failure to bring or maintain the electrodes in contact with the target tissue may result in the RF energy not reaching the tissue in sufficient quantities to effect ablation. Only limited electromagnetic flux in a bipolar approach may reach the tissue when the electrode is non-contacting. In a unipolar approach, the RF energy may spread out too much from the non-contacting electrode before reaching the tissue so that a larger surface area is impacted by the flux resulting in each unit volume of tissue receiving that much less energy. In both cases, the process of raising the tissue temperature to the ablation point may require a much greater time period, if it can be performed at all. Where the electrodes have temperature sensors and those sensors are not in contact with the tissue, they may not sense the actual temperature of the tissue as fluids flowing around the non-contacting electrode may lower the temperature of the electrode and the temperature sensed by the sensors.
In some procedures, such as where a longer atrial lesion is required, the lesion produced by a single electrode in a unipolar arrangement is not sufficient. To this end ablation catheters have been designed. In one catheter an electrode device having four peripheral electrodes which extend from a retracted mode is used. See U.S. Pat. No. 5,500,011 to Desai. When extended, i. e., fanned out, the four peripheral electrodes and the central electrode form an electrode array that covers a larger surface area of the tissue than a single electrode. However, there remain some difficulties in manipulating such a device so that when expanded, all electrodes are in contact with the endocardium. An "end on" approach is required such that the end of the catheter, on which all five electrodes are mounted, is in intimate contact with the target tissue.
The effectiveness of the above-described technique is further limited by the mechanical configuration of the electrode device. When used for ablation, an electrode device is typically part of a catheter system. Accordingly, it is desirable to minimize the diameter of the electrode device during introduction to and withdrawal from the patient to allow for its use within a catheter and to lessen trauma to the patient. However, it is desirable to obtain a relatively large expandable size to obtain a larger ablation size. Therefore, electrode devices having peripheral expandable electrodes must be configured so that the peripheral electrodes are expandable to a large size yet are retractable to as small a size as practical. Such requirements pose design and manufacturing difficulties and an electrode device configured as such is susceptible to malfunction in that the peripheral electrodes may be damaged or break off as they are extended from a retracted mode or vice versa. Further considerations are the undesirable complexity and increased manufacturing cost associated with an expandable catheter.
In other techniques, used in the treatment of atrial fibrillation, a plurality of spaced apart electrodes are located at the distal end of the catheter in a linear array. RF energy is applied to the electrodes to produce a longer lesion. With such a linear array, intimate contact between each electrode and the target endocardial tissue is more difficult to maintain in the heart due to the irregular heart surface shape and the constant movement of the heart. The lesion produced may have discontinuities unless steps are taken to maintain contact. These lesions may not be sufficient to stop the irregular signal pathways and arrhythmia may reoccur. In an attempt to ensure intimate contact between the electrode and the target tissue the distal end of the catheter may have a preformed shape. For example, see U.S. Pat. No. 5,617,854 to Munsif, in which the catheter is made of a shaped-memory material, e.g. nitinol, and formed in a specific shape. During use, the catheter is deformed and introduced through an introducer sheath to the heart where ablation is to occur. Once in position, the catheter is reformed into its specific shape when heated to body temperature or when a current is passed through the shaped-memory material. If the shaped memory of the catheter matches the curvature of the biological cavity, there is more intimate contact between the electrode and the tissue and a more continuous lesion is formed. However, this is somewhat unlikely because the heart surface is irregular. If a given shaped catheter does not conform to the shape of the biological site to be ablated a different catheter having a different preformed shape must be used. Requiring a collection of preformed-shaped catheters, as such, is economically inefficient.
Hence, those skilled in the art have recognized a need for a structurally stable minimally invasive ablation apparatus that is capable of controlling the flow of current through a biological site so that lesions with controllable surface and depth characteristics may be produced and the ablation volume thereby controlled. Additionally, a need has been recognized for providing a catheter carrying a plurality of electrodes in its distal end region which is capable of conforming to various curvatures of the biological site so that intimate contact may be maintained between the electrodes and the site. The invention fulfills these needs and others.